1. Field of the Invention
The present invention relates to a light-emitting element array and an image forming apparatus, and specifically to a light-emitting element array which has a high light output intensity and operates with time division driving, and an image forming apparatus using the light-emitting element array.
2. Description of the Related Art
A light-emitting element array in which several thousand light-emitting diodes are arranged is used for an exposure light source of an electrophotographic printer. For example, the array is produced by forming an element structure each including several AlGaAs layers on a compound semiconductor substrate made of GaAs or the like and forming it to an array state (Japanese Patent No. 3185049).
When the light-emitting element array is used for a printer, it is necessary to determine an element size and an element interval according to desirable printing resolution. For example, in a case of 600 dpi, it is necessary to reduce the element size to at least a square of 40 μm or less and reduce the element interval to approximately 40 μm. In a case of 1200 dpi, half of each of the element size and the element interval is required. When the light-emitting element array is used as a printer light source, it is necessary to separately drive the light-emitting elements. In actuality, a time division driving method is used as a driving method to reduce the necessary number of electrodes for light emitting elements, the necessary number of driver IC chips, and the necessary number of wires for wire bonding, thereby suppressing an increase in cost (Japanese Patent No. 3340626).
A metal reflection layer is provided under a light-emitting element portion to improve extraction efficiency, thereby increasing light output intensity (Japanese Patent Application Laid-Open No. 2005-197296). When the light output intensity becomes higher, high-speed printing can be performed. Also, the light output intensity reaches a desirable intensity at a small current value, so that it is possible to realize a high-definition printer light source having small adverse effects caused by heat generation, such as poor device characteristics, poor device lifetime, or larger deviation of light emitting area.
Up to now, when the metal reflection layer is provided under the light-emitting element portion, light traveling to a substrate side can be reflected on the metal reflection layer and can be extracted from an element surface, thereby increasing the light output intensity. Usually a metal layer has a high conductivity, so that a method of easily performing current injection in a vertical direction by actively using the conductivity of the metal layer is used.
When a plurality of light-emitting elements are arranged to produce an array, for example, a method of forming a single n-side electrode as a common electrode and forming p-side electrodes corresponding to all the light-emitting elements to drive the light-emitting elements is considered. This is a fundamental driving method which is normally called a static driving method. However, for example, when a resolution of 1200 dpi is to be realized in a case of an A4 size, the number of elements is ten thousand or more, and an element density becomes very high. Therefore, there is no space for separately placing the p-side electrodes, thereby resulting in a problem that it is difficult to perform wire bonding for connection with driver IC chips.
Even in the case of the resolution of approximately 600 dpi, 5000 or more elements are required to realize the A4-size and thus 5000 or more driver IC chips and 5000 or more wires for wire bonding are required. Therefore, a method of reducing these numbers is desired for a reduction in cost.
In order to solve the above problems, time division driving is employed. This is also called dynamic driving. According to this driving, although the light-emitting elements are driven in time division, the number of electrodes necessary to drive all the elements can be reduced. In this case, electrode wiring is matrix wiring. In the matrix wiring, not a single common electrode is provided, but a plurality of common electrodes (for example, n-side electrodes) are formed, provided that each of common electrodes is formed per block including a plurality of light-emitting elements. On the other hand, it is unnecessary to independently place an electrode having the other conductive type (for example, p-side electrodes) for all the elements. When the number of p-side electrodes to be provided is equal to the number of all the elements included in the block in which the common electrode is formed, all the elements can be fundamentally driven. In this case, a p-side electrode is connected with not only one element in one block but also other elements included in other blocks which are different from the one block. That is, the plurality of elements are driven by using one p-side electrode.
FIG. 11 is an explanatory diagram illustrating time division driving. Light-emitting elements (light-emitting element portions) L1 to L9 are divided into three blocks. A first block includes three light-emitting elements L1 to L3, a second block includes three light-emitting elements L4 to L6, and a third block includes three light-emitting elements L7 to L9. N-side electrodes 11-1, 11-2 and 11-3 are provided for the light-emitting elements included in the respective blocks. P-side electrodes 17-1, 17-2 and 17-3 are provided for the light-emitting elements located at the same arrangement position in each of the blocks. In the first block including the light-emitting elements L1 to L3, while the n-side electrode 11-1 is selected by a switch and maintained at a ground (GND) potential, one of the p-side electrodes from 17-1 to 17-3 is selected by the other switch to supply a current to a target light-emitting elements from which light is to be emitted. Similarly, in the second block including the light-emitting elements L4 to L6, while the n-side electrode 11-2 is selected, one of the p-side electrodes from 17-1 to 17-3 is selected by the other switch. Similarly, in the third block including the light-emitting elements L7 to L9, while the n-side electrode 11-3 is selected, one of the p-side electrodes from 17-1 to 17-3 is selected by the other switch. The time division driving is performed by the above-mentioned operation.
Hereinafter, the structural examples of a light-emitting element array using AlGaAs for static driving and time division driving will be described. FIGS. 12 and 13 are a cross sectional view and a plan view, respectively, which illustrate a light-emitting element array capable of performing static driving. In this example, nine light-emitting elements (light-emitting element portions) in total are provided. Nine separate p-side electrodes 17 and one common n-side electrode 11, that is, ten electrodes in total are required. In FIGS. 12 and 13, an n-type AlGaAs layer 13, AlGaAs quantum well active layers 14, p-type AlGaAs layers 15, p-type GaAs contact layers 16, and the p-side electrodes 17 are formed on an n-type GaAs substrate 12. For element separation, separation grooves (element separation grooves) 18 are formed so as to reach the n-type AlGaAs layer 13. In a region 21, an insulating film 19 is formed on a portion of the n-type AlGaAs layer 13 exposed by etching for electrical insulation. In a region 22, a portion of the insulating film 19 is formed on the p-type GaAs contact layers 16 left without being etched. In each of light-emitting regions 23, a portion of the p-side electrode 17 is in direct contact with an upper surface of the p-type GaAs contact layer 16. When a current is injected from the p-side electrode, light is emitted from the region 23. The insulating film 19 is provided in the regions 21 and 22 and not provided in the regions 23, so that a current can be injected to only a necessary light-emitting region 23 through a corresponding p-side electrode 17. Each of the separation grooves 18 is provided to electrically separate adjacent light-emitting elements from each other.
FIGS. 14 and 15 are a cross sectional view and a plan view, respectively, which illustrate a light-emitting element array capable of performing time division driving. The same constituent members as the members illustrated in FIGS. 12 and 13 are indicated by the same reference numerals. In the ⅓-time division driving as illustrated in FIGS. 14 and 15, a matrix wiring of 3×3 is employed as described with reference to FIG. 11, all pixels can be driven by three common n-side electrodes and three common p-side electrodes, that is, six electrodes in total.
According to this method, when the number of time division is increased, the number of electrodes can be significantly reduced. However, unlike the static driving, it is necessary to form a plurality of common electrodes. In a normal case, a semi-insulating substrate 31 is used and separation grooves 32 which reach at least the surface of the substrate 31 are formed, so that the plurality of common electrodes can be relatively easily formed. The separation grooves 32 are provided to electrically isolate the light-emitting elements from one another for each block. As described above, in the case of the light-emitting element array using the metal reflection layer, an increase in light emission intensity is expected by the metal reflection layer. When the electrical isolation for each block is required for time division driving, the conductivity of the metal reflection layer becomes a problem for the purpose. Therefore, as illustrated in FIG. 16, it is necessary to form the separation grooves which reach not only a semiconductor layer but also a metal reflection layer 52. In the case of the separation of only the semiconductor layer, an etching process is performed one time. However, in the case of the separation of the metal reflection layer, a different etching process is normally further necessary, with the result that there is a problem that an increase in cost and a reduction in yield occur.